The segmentation pattern of many higher animals constitutes a well-defined set of endpoints or levels of development for analyzing the genetic control of development. In Drosophila we have found that much of the basic body segmentation of the thoracic and abdominal (AB) segments is under the control of a giant cluster of genes known as the bithorax gene complex (BX-C). This complex comprises a minimum of nine genes which appear to code for substances which control specific body segment transformations. Genes in the distal portion of BX-C "abdominalize" segments AB2 to AB8, inclusive, while genes of the proximal portion abdominalize AB1 and modify the third thoracic segment so that it bears halters instead of wings. For four of the genes of the complex, cis-dominant regulatory-like elements have been identified. The entire complex acts as if it is under negative control mediated by a repressor coded for by another gene, Polycomb (Pc ion). Three major rules govern the behavior of the bithorax genes during early development. (1) The more posterior the segment the more genes of the complex become derepressed; (2) genes once derepressed in a given segment tend to be derepressed in all segments posterior thereto; and (3) the more proximal a gene is in the complex the more likely (with one exception) it is to be derepressed. The way in which the complex controls normal development can perhaps best be understood if there are two gradients: a gradient along the embryo in a substance produced by Pc ion and a gradient along the chromosome in the affinity of the cis-regulatory elements for that substance.